1. Field of the Invention
The present invention relates in general to a method and apparatus for hydraulic actuation of a downhole tool and, in particular, to an apparatus and method for actuating one or more functions of a downhole tool with a dampered drop plug.
2. Brief Description of Related Art
A variety of tools exist to perform downhole functions in a well. Some tools may be actuated in response to mechanical movement or manipulation of the drill pipe, including rotation. Others may be actuated by dropping a ball or dart into the drill string, then applying fluid pressure to the interior of the string after the ball or dart lands on a seat in the tool. The tool may be attached to the liner hanger or body of a running tool by threads, shear elements, or by a hydraulically actuated arrangement.
Oil and gas wells are conventionally drilled with drill pipe to a certain depth, then casing is run and cemented in the well. The operator may then drill the well to a greater depth with drill pipe and cement another string of casing. In this type of system, each string of casing extends to the surface wellhead assembly.
In some well completions, an operator may install a liner rather than an inner string of casing. The liner is made up of joints of pipe in the same manner as casing. Also, the liner is normally cemented into the well. However, the liner does not extend back to the wellhead assembly at the surface. Instead, it is secured by a liner hanger to the last string of casing just above the lower end of the casing. The operator may later install a tieback string of casing that extends from the wellhead downward into engagement with the liner hanger assembly.
When installing a liner, in most cases, the operator drills the well to the desired depth, retrieves the drill string, then assembles and lowers the liner into the well. A liner top packer may also be incorporated with the liner hanger. A cement shoe with a check valve will normally be secured to the lower end of the liner as the liner is assembled. When the desired length of liner is reached, the operator attaches a liner hanger to the upper end of the liner, and attaches a running tool to the liner hanger. The operator then runs the liner into the wellbore on a string of drill pipe attached to the running tool. The operator sets the liner hanger and pumps cement through the drill pipe, down the liner, and back up an annulus surrounding the liner. The cement shoe prevents backflow of cement back into the liner. The running tool may dispense a wiper plug following the cement to wipe cement from the interior of the liner at the conclusion of the cement pumping. The operator then sets the liner top packer, if used, releases the running tool from the liner, and retrieves the drill pipe.
For tools that are set by dropping a ball or dart into the drill string, such as the above described liner hanger, a seat in the running tool couples to the running tool by shear elements downhole from the hydraulically actuated tool. The shear elements are chosen to fail at a pressure greater than the pressure needed to operate the tool. The ball drops into the drill string to land on the seat in the running tool. Once landed, fluid pumps into the drill string, increasing the pressure within the drill string above the seated ball. Once the fluid pressure reaches a predetermined pressure, the tool actuates. Fluid pressure continues to increase until the shear pressure of the seat is reached. At this point, the shear elements of the seat fail, and the ball and seat fall, allowing the pressurized fluid to flow down the well.
In some instances, the drop ball will also be used to actuate a second hydraulically actuated tool. In these examples, a second seat in the running tool couples to the running tool axially below the first seat. Again, the second seat couples through the use of shear elements. Preferably, when the first shear elements fail, the ball drops to the second seat, again blocking the flow of fluid into downhole elements below the ball. Fluid continues to pump into the drill string, raising the pressure behind the ball until the second function actuates. Practically, when the first shear elements fail, the ball drops to the second seat, and the fluid pressure behind the ball acts as a water hammer on the second shear elements. The weight of the fluid column above the ball suddenly lands on the seat shear elements. The force exerted by the suddenly falling fluid often exceeds the shear strength of the second shear elements. This then causes the second shear elements to fail prior to activation of the second hydraulically activated tool. Therefore, there is a need for a drop ball system for actuating multiple hydraulically activated tools that overcomes the water hammer shear problems of current drop ball systems.